Well this will all depend on when Thanos is used as a main antagonist for the Avengers and with Guardians of the Galaxy set to debut before the next Avengers film along with the possibility that Thanos will be first better established in that film, that makes me think that Thanos will be a force to be actively reckoned with in the upcoming Avengers Sequel.

Plus, I personally still don't think that we'll have Surtur as the main villain for Thor 2 and that he'll be fully revealed and used in Thor 3.